1. Field of the Invention
The present invention relates to a mounting structure for connecting, to an external circuit board, a high-frequency package in which is mounted a semiconductor element that operates in a high-frequency region of from a microwave band to a millimeter-wave band (hereinafter often referred to as high-frequency element). More specifically, the invention relates to a mounting structure in which a high-frequency signal can be transmitted with a small loss.
2. Description of the Prior Art
In a high-frequency package mounting a high-frequency element, in general, the high-frequency element is held in a cavity formed in the surface of a dielectric board, and the cavity is air-tightly closed with a closure. On the surface of the dielectric board is further formed a signal transmission line such as a strip line electrically connected to the high-frequency element, and the high-frequency signals are input to, or output from, the high-frequency element through the signal transmission line.
In the high-frequency package, furthermore, the signal transmission line on the surface of the dielectric board is drawn toward the outside of the cavity from the inside of the cavity through the side wall of the closure of the cavity. Furthermore, the line that is drawn is folded on the side surface of the dielectric board, drawn along the bottom surface of the dielectric board, and is connected to a terminal for connection to an external circuit board (mother board). Moreover, a via-hole conductor is formed to penetrate through the dielectric board, and the signal transmission line on the surface of the board is connected to the connection terminal on the bottom surface of the board through the via-hole conductor. That is, the connection terminal provided on the bottom surface of the dielectric board is connected to the wiring layer of the external circuit board using an adhesive such as solder, in order to mount the high-frequency package on the external circuit board. Owing to such a mounting structure, the high-frequency signals are input to the high-frequency element from the external circuit board or are output from the high-frequency element to the external circuit board through the signal transmission line.
In the above-mentioned structure for mounting the high-frequency package, however, there occurs a problem in that the transmission characteristics are deteriorated due to mismatching of the impedance, reflection loss and radiation loss of the transmission signals at a portion where the signal transmission line is passed through the side wall of the closure. Even when the signal transmission line is connected to the connection terminal by using the via-hole conductor that penetrates through the dielectric board, the transmission characteristics are still deteriorated due to mismatching of the impedance caused by the via-hole conductor.
In order to solve the above-mentioned problems, Japanese Unexamined Patent Publications (Kokai) Nos. 250911/1996, 250913/1996 and 186268/1997 propose high-frequency packages in which the signal transmission line is also formed on the bottom surface of the dielectric substrate, and is coupled, relying upon the electromagnetic coupling, to a signal transmission line that is connected to the high-frequency element on the surface of the dielectric board.
Referring to FIG. 11 which is a sectional view schematically illustrating a high-frequency package of a prior art, a cavity 43 is formed in the surface of a dielectric board 41, the cavity 43 being air-tightly sealed with a closure 42 made of an electromagnetic wave-shielding material such as a metal, and a high-frequency element 44 is mounted in the cavity 43. An input line 45a and an output line 45b are formed on the surface of the dielectric board 41, and are electrically connected to the high-frequency element 44. Furthermore, an input line 46a and an output line 46b are formed on the bottom surface of the dielectric board 41, and a ground layer 48 having a pair of slots 47a, 47b is formed in the dielectric board 41. As will be obvious from FIG. 11, the ends of the input lines 45a and 46a are opposed to each other with the slot 47a interposed therebetween, and are electromagnetically coupled together via the slot 47a. The ends of the output lines 45b, 46b are opposed to each other with the slot 47b being interposed therebetween, and are electromagnetically coupled together via the slot 47b.
Furthermore, connection portions 49a, 49b are formed at the ends of the input line 46a and the output line 46b on the bottom surface of the dielectric board 41, and the high-frequency package is mounted on the external circuit board by using these connection portions. That is, the external circuit board has a wiring layer with an input line 51a and an output line 51b formed on the surface of the insulating board 50. The high-frequency package is mounted on the external circuit board by connecting the connection portions 49a and 49b to the input line 51a and to the output line 51b by using an electrically conducting adhesive such as a solder as designated at 52a, 52b.
According to the above mounting structure, therefore, high-frequency signals are input to the high-frequency element 44 through the line 51a-line 46a-slot 47a-line 45a, and high-frequency signals are output from the high-frequency element 44 through the line 45b -slot 47b-line 46b-line 51b.
With the structure for mounting the high-frequency package according to the above-mentioned prior art, however, high-frequency signal transmission characteristics are deteriorated when the insulating board constituting the external circuit board has a dielectric constant of not smaller than 7. Therefore, the above-mentioned mounting structure could not be used in high-frequency regions. That is, the high-frequency package of the above-mentioned prior art could be mounted on the external circuit board constituted by an insulating board having a particular dielectric constant only.
With the mounting structure shown in FIG. 11, furthermore, the high-frequency element mounted on the package was not allowed to exhibit its characteristics to a sufficient degree particularly when the signals in the millimeter-wave band are to be treated. For example, when the transmission characteristics of millimeter-wave signals are measured between the line 51a and the line 51b at the ends of the input and output transmission lines, it is learned that the insertion loss is so great that isolation characteristics are not maintained to a sufficient degree. The isolation characteristics between the signal transmission lines tend to decrease, too, even when the package is connected by soldering to the external circuit board at positions 52a, 52b in the regions close to the electromagnetically coupled portions through the slots.
In the above-mentioned package, furthermore, a via-hole conductor and a metal band may be provided in the dielectric board in order to radiate heat to the external side from the portion on where the high-frequency element is mounted or in order to enhance the isolation characteristics. When they are provided in a region close to the slot, however, resonance occurs to deteriorate transmission characteristics between the signal transmission lines.